If there is a problem with nitrifying bacteria when the pH drops below 6 everyone who has that low a pH will be having ammonia problems or nitrite problems with their fish. How many reports have you seen of that? I don't recall even one, where that was clearly the problem. The bacteria that convert ammonia to nitrates is naturally occurring, and will occur in spite of what you do to your water, assuming the water is good for fish to live in. I skimmed that link in "oscarfish", but I have no reason to believe that the author is an expert (or not an expert, for that matter), so I have to fall back on experience on this.